Use Tom Hirschberger’s fork https://github.com/Tom-Hirschberger/MMM-OpenWeatherForecast it works great!
You need to put in payment information to subscribe to the new 3.0 API, but once you do that, it will work (it took about 30 minutes or so for it to work properly on my end) Thank you to Tom for providing this updated version! Got the email about the old API being discontinued, and I knew I’d need to look around for a working fork.